Project 3 is a clinically oriented examination of repolarization changes that precede ventricular tachycardia and ventricular fibrillation. Experiments will be performed in both dogs and patients. Project 3 contains three subprojects: Project 3.1, Repolarization Changes Immediately Preceding Ischemic Sudden Death: Project 3.2, Repolarization Changes Immediately Preceding Sustained Ventricular Tachycardia; and Project 3.3, Repolarization Change as a Predictor of Outcome in the ESVEM Trial. The three subprojects are unified by a general hypothesis: Repolarization changes precede and may be causative of ventricular tachyarrhythmias. In Project 3.1 we will examine the nature and extent of repolarization changes during acute myocardial infarction as reflected by the QT interval, the activation-recovery interval and the QRST integral. We will determine the extent to which a change in resting potential is responsible for alteration in QRST integral. A system for recording repolarization changes chronically in man will be validated. In Project 3.2 we will measure repolarization changes occurring immediately before onset of sustained ventricular tachycardia in patients enrolled in the ESVEM Trial using an automated analysis of Holter tape recordings. In Project 3.3 we will use the same capability and patient population to evaluate the prognostic significance of variability of repolarization duration, and the relationship between change in that variability.